[fitsbits] XTENSION = 'FITS' proposal

[Steve Allen]

On Mon 2002-04-22T13:50:39 -0400, Perry Greenfield hath writ:
> William Pence writes:
> > only difference is that in one case the length of the group is given by
> the
> > NAXIS1 keyword in the 'FITS' extension, and in the other case the end of
> the
> > group is marked by the GROUP_END extension.
> >
> I basically agree. There is a difference in how easy it is to skip over
> the group in one case versus the other, but I don't know if I would consider
> that a major issue (it might be if one is trying to access a single item
> nested a couple levels down in a structure with many components at each
> level).

As discussions for XTENSION = 'FITS' continue it should be considered
whether the recursive encapsulation should permit anything to be within
anything, or if there should be restrictions on the hierarchical
structure.  Restrictions on the structure could be used to define a
namespace for the access of the HDUs.  This would make the job of the
toolkit designer clearer, albeit harder.

A simple example: the specification could state that any given value
of EXTNAME can only occur in one HDU at a given recursion level.  Then
each HDU could be named as
	fitsfile.top_level_extname.next_level_extname.lower_level_extname
and so on.  Or the specification could permit uniqueness of the
combination of EXTNAME and EXTVER, and then the structure naming
could be array-like
	fitsfile.top_level_extname.next_level_extname[2].lower_level_extname

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
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